1. Field of the Invention
The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and an intensifying screen. As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as the radiographic intensifying screen employed in the conventional radiography.
When the radiation image recording and reproducing method is practically carried out, the radiation image storage panel is repeatedly used in a cyclic procedure comprising steps of exposing the panel to a radiation (i.e., recording a radiation image), irradiating the panel with stimulating rays (i.e., reading out the recorded radiation image), and exposing the panel to light for erasure (i.e., erasing the remaining energy from the panel). The panel is moved from one step to the next step, being held by the transfer means such as rolls and endless belt, and after one cycle is finished, the panel is usually piled upon other panels and stored in the radiation image recording and reproducing apparatus having said cyclic system.
From the viewpoint of flexibility required in transferring the radiation image storage panel, plastic films such as a polyethylene terephthalate film or various papers are desirably employed as a support material.
In the repeated use of the radiation image storage panel involving transferring and piling, a front surface (phosphor layer-side surface or protective film-side surface) and a back surface (support-side surface) of the panel, both of which are usually composed of polymer materials or papers, are apt to be electrically charged with minus and plus, respectively, by physical contact such as rubbing of a front surface of a panel against a back surface of another panel, rubbing of a front surface or back surface of a panel against an edge of another panel, or rubbing a panel surface against transfer means such as roll and belt, when the panel is piled on the other panels or moved from the pile to the transfer system. Thus electrically charged panel causes various problems in performing the radiation image recording and reproducing method.
For instance, the electrically charging of the panel surfaces tends to bring about adhesion of the front surface of a panel to the back surface of another panel, so that it is difficult to separate the panels in the direction parpendicular to the panel surface and the combined two panels are moved together to the transfer system, whereby the subsequent procedure cannot be performed normally. Further, the dust in the air also tends to deposit to the charged panel surface. A radiation image is generally read out from the phosphor layer-side surface of the panel, and the dust deposited panel surface causes scattering of the stimulating rays in the read-out procedure. As a result, the quality of resulting image tends to be extremely deteriorated. In addition to these drawbacks, the sensitivity of the panel decreases, a noise such as static marks is produced on the resulting image, and shocks are given to handlers thereof, because of the discharge of the panel.
There has been known the art that a light-absorbing material such as carbon black is incorporated into the support made of plastic film for improving the image quality such as sharpness. However, the amount of the incorporated carbon black for said purpose is very small, and the electrification of the panel cannot be prevented sufficiently even if the panel is provided with the support containing carbon black. For example, a commercially available panel provided with the support containing carbon black (trade name: Fuji CR Imaging Plate ST, available from Fuji Photo Film Co., Ltd.) has an electric resistivity of not lower than 10.sup.15 ohm on the support-side surface. It has been also proposed to incorporate a light-reflecting material such as titanium dioxide, aluminium oxide, silicon oxide or zinc oxide into the plastic support for enhancing the sensitivity of the panel, as described in Japanese patent application No. 57(1982)-182111. Further, Japanese Patent Publication No. 56(1981)-12600 discloses that a light-reflecting layer comprising a white pigment such as titanium white, white lead, zinc sulfide, aluminium oxide or magnesium oxide is provided between the support and the phosphor layer for enhancing the sensitivity of the panel.